Unlocking the Potential: The Schematic Diagram of Dark Excitons in Stacked Semiconductor Materials

2023-10-12 02:03:39

Schematic diagram of dark exciton generated when two single-layer semiconductor materials are stacked and exciton generated

[충청뉴스 이성현 기자] Next-generation semiconductors that utilize quasiparticles ‘exciton’ instead of electrons are expected to be released.

At the Institute for Basic Science (IBS), the research team of Director Younghee Lee of the Nanostructure Physics Research Center, through joint research with the research team of Seokjun Yoon, a fellow at the Oak Ridge National Laboratory in the United States, and the research team of Professor Ermin Malik at Philips-Marburg University in Germany, developed a device that stacks different semiconductor materials. It was announced that ‘dark exciton’ was detected for the first time.

Semiconductor materials have two bands where electrons can exist. The lower band filled with electrons is called the ‘valence band’, and the upper band empty of electrons is called the ‘conduction band’. When external energy is received, electrons in the valence band are excited to the conduction band, and the empty space where the electron disappears is called a hole.

Holes pair with electrons that have risen to the conduction band to form quasiparticles called exciton.

Exciton is divided into ‘bright exciton’ and ‘dark exciton’. Bright exciton is already used in quantum dot displays (QLED) due to its characteristics of being absorbed by light and is easily detected, while dark exciton has a longer lifespan than bright exciton. It has the advantage of being longer and more stable, making it easier to use as a semiconductor, but it hardly absorbs light, making it difficult to detect.

For this reason, only the operating principle in a single material has been identified so far, and it has not been revealed exactly how it is expressed in a heterojunction device made of multiple materials stacked similar to the actual semiconductor device environment.

Accordingly, the research team confirmed the behavior of exciton by irradiating laser light with a TMD heterojunction device in which different types of TMD are stacked on a sheet of transition metal disulfide compound (TMD). As a result, it was discovered that dark excitons appear and disappear depending on the order in which the TMD materials are stacked. In particular, in the case of the upper TMD material, it was confirmed that dark excitons always appear regardless of the stacking order.

Furthermore, it was discovered that the dark exciton becomes brighter when the intensity of the applied light decreases, making it possible to control the intensity of the dark exciton according to the intensity of light. This opens up the possibility of improving the efficiency of broadband solar cells by using dark excitons in the ‘energy filter’ that distinguishes energy or color and the ‘power filter’ that controls the intensity of light.

Director Lee Young-hee said, “With the discovery of dark excitons in heterojunction materials for the first time, we look forward to the application of next-generation optical semiconductors with the power and energy filter functions expected in the next generation.”